As a method for using acrylic resins in dental applications, there is known a method in which a polymer powder of methyl methacrylate and a liquid monomer of methyl methacrylate are mixed with each other in a weight ratio of about 2/1, the mixture is allowed to stand for 10 minutes or longer to be a dough, and the dough is then filled in a mold or gypsum mold for molding a denture or denture base, or cavities of teeth, thereby allowing it to polymerize and cure. And as a polymerization means, there is known a heat polymerization method in which the above-described polymer powder is previously added with benzoyl peroxide as a polymerization initiator, and after filling the dough in the mold, it is heated at 60.degree. C. or higher, thereby decomposing the benzoyl peroxide to start the polymerization. Since in this method, moldings with a high strength can be obtained, this method is employed for molding a denture and the like.
In addition, there are also known a low temperature polymerization method in which while adding benzoy peroxide to the polymer powder, the liquid monomer is added by a tertiary amine, and the tertiary amine is brought into contact with benzoyl peroxide upon mixing of the liquid monomer with the polymer powder, thereby decomposing the benzoyl peroxide to start the polymerization; a photopolymerization method in which a photo-sensitizer is used in place of the tertiary amine in the above-described autopolymerization method, the photo-sensitizer is decomposed upon irradiation with ultraviolet rays or visible rays, thereby decomposing the benzoyl peroxide with the decomposition product to start the polymerization; and the like. Since these methods do not need heating, they are employed for applications of fillers or cements for filling in or applying on cavities of teeth and the like. In the foregoing low temperature polymerization method or photopolymerization method, by attaching the polymer powder to a brush tip soaking up the liquid monomer, the mixing of the liquid monomer with the polymer powder is carried out, and a spherical viscous mixture at the brush tip is filled in or applied on cavities of teeth, thereby molding it in an arbitrary shape.
However, in the above-described method, even when any of the heat polymerization method, low temperature polymerization method, or photopolymerization method is employed for the polymerization, the curing shrinkage caused by the polymerization of the monomer is as high as about 0.5 to 0.6% in terms of the curing shrinkage rate. For example, in case that a denture base is prepared, there was an inconvenience that a space is formed in the back surface (mucous membrane) side, resulting in poor fitness. And an attempt in which the polymerization is carried out from the back surface side to prevent the shrinkage in the back surface side, thereby preventing the poor fitness of the denture base is being made. In this case, however, there was still a problem that since shrinkage occurs in the front surface side opposite to the back surface side, the biting becomes worse.